Silos are commonly used for storing granular material such as sand, cement, salt, fertilizer, and the like. The silos may be installed at a permanent site such as a concrete plant or the like, but often it is desired to store granular material at temporary work sites such as oil wells and the like. Portable storage silos are well known in various industries for temporary on-site storage of various granular materials. The empty silos are transported to a work site and supported suitably to accommodate the weight of the granular material being stored, and then the silo filled with granular material.
These silos are commonly oriented vertically to maximize the quantity of material stored above a hoppered bottom such that the dry material will flow out by gravity through a discharge opening with a gate that can be opened or closed to control flow out of the silo. Portable silos are often transported to the work site in a lowered horizontal transport orientation, and then raised to a vertical working orientation at the work site for similar reasons.
It is common for such storage silos, permanent or portable, to include a loading conduit to allow connection to discharge pipes of a pneumatic conveyor on a transport vehicle such that granular material can be conveyed into the silo by the pneumatic conveyor. Such pneumatic conveyors typically comprise a blower operative to create an air stream in a conveyor pipe connected to a storage vessel on the transport vehicle to receive a flow of granular material that mixes with the air stream and is carried along the conveyor pipe to an output end thereof.
A lower input end of the loading conduit is connected to the output end of the conveyor pipe, and the air stream and granular material mixed therein move through the loading conduit to a discharge end of the loading conduit located in an upper portion of the storage silo where the granular material falls out of the air stream into the silo. The top of the silo includes vents to allow the air stream entering the silo to escape.
When transferring granular material into such silos a considerable amount of dust is often generated, depending on the nature of the granular material. This dust has conventionally simply been vented out into the atmosphere with the air stream however more recently it has been recognized that the dust presents a health hazard to workers at the site, and efforts have been made to control the dust by filtering the escaping air stream.
Dust similarly is generated when the granular material flows out of the discharge opening. The granular material often discharges through a chute which directs the material into a conveyor or like target location. The flow of granular material along the chute, and the granular material landing on the target location generate considerable dust as well. Granular material is often drawn out of these silos at high rates, for example 500 or more cubic feet per minute, such that when drawing material out of the silo air passes from the atmosphere through the vents into the silo at a rate substantially the same as the rate of material removal.